Controlling outdoor lights with timers and photocells

Outdoor lights switched by hand are either forgotten or left burning all night. How photocells, timers and a combination of the two work, plus installation details.

Yard, porch and garden path lights tend to end up in one of two situations. Either you forget to switch them on in the evening and step out into a dark yard, or they are switched on and burn until morning, sometimes all day. The fix in both cases is automation: a photocell and a timer. They are simple devices, but chosen badly or mounted in the wrong place, they do not do their job.

Photocell: reacting to darkness

A photocell is a sensor that measures light level. In the evening, when light falls below a threshold, it closes the circuit and the lamp comes on; in the morning, when it gets light, it opens it again. Its main advantage is that it follows the seasons: it switches on early in winter and late in summer without any adjustment.

Its drawback follows from the same principle: it keeps the light on all night. That is needed in some places, such as a gate or steps. But lighting an empty garden path at two in the morning is usually unnecessary.

Most photocells have a sensitivity adjustment that sets how early in the dusk the light comes on. Setting the threshold slightly lower helps stop the light switching on during a dark, cloudy day.

Timer: control by the clock

A timer switches the light on and off at set times. Mechanical timers are simple and cheap; digital ones let you program different schedules for different days of the week. The weakness is the seasons: at 17:30 in December it is already dark, while in July the sun is still shining. The program needs updating several times a year.

Astronomical timers solve this: you enter the location and the device calculates sunset and sunrise itself. Such a timer follows the seasons like a photocell, without needing a sensor.

Best of both: combining them

In practice the most convenient arrangement uses a photocell and a timer together:

• when darkness falls, the photocell switches the lights on;
• at a set time at night, for example after the household goes to bed, the timer switches the general yard lighting off;
• an essential point, such as the lamp above the gate, stays on until morning or is moved to a motion sensor;
• when it gets light, the photocell switches everything off.

This way the lights run only when they are actually useful, and nobody has to remember them. Some ready-made devices combine a photocell and timer in one housing.

Where to mount the photocell

Half of a photocell’s performance depends on its position. Typical mistakes:

• The sensor sees the lamp it controls. The lamp comes on, the photocell decides it is daylight and switches off, then switches on again in the dark. The result is a light that cycles on and off. Mount the sensor where the lamp’s light cannot reach it.
• A streetlight or a neighbour’s floodlight shines on the sensor. The yard light may never come on.
• The sensor sits in deep shade. Under dense trees or in a narrow passage it may “see” darkness in daytime and switch the light on.
• The sensor is exposed to rain and dirt. A layer of dust blinds it, so wipe it occasionally.

The best spot is usually a north-facing section of wall with a view of open sky and no direct sun or artificial light.

Lamp and device compatibility

Using LED bulbs with a photocell or timer outdoors is usually trouble-free, but check a few points. Electronic photocells and timers have minimum and maximum load limits, and when many LED lamps share one relay, the inrush current can stress the contacts. Look for the LED load limit in the device’s specifications.

Anything mounted outside has to carry an IP rating that matches its exposure to rain and dust. Use bulbs inside closed, weatherproof fittings. Leave the wiring to an electrician; for outdoor circuits a circuit breaker and residual current protection are especially important.

Seeing the saving

A 15 W lamp on a photocell alone can run 13–14 hours a day in winter. If a timer switches it off at midnight, the running time drops by several hours. Work out the hours for your own yard and enter them into the savings calculator on this site to see the difference in kWh.

Enclosed yard lights on a timer or photocell can take E27 Zəfər İşığı LED bulbs, which come in several wattages; choose a bulb within the fitting’s maximum wattage.